Mechanisms for Lagged Atmospheric Response Mechanisms for Lagged Atmospheric Response to ENSO SST Forcingto ENSO SST Forcing

Hui SuHui Su****, J. David Neelin, J. David Neelin**** and Joyce E. Meyerson and Joyce E. Meyerson**

Dept. of Atmospheric Sciences*, Inst. of Geophysicsand Planetary Physics**, U.C.L.A.

• Tropical Tropospheric Temperature Anomalies (<T>´) Lag ENSO SST Anomalies by 1-3 months

• QTCM Experiments with Prescribed SST and with a Slab Mixed-layer Ocean ModelPhase and Amplitude of <T>´ Dependence on:

Mixed-layer DepthENSO SST FrequencyFraction of Mixed-layer ocean Region

• A Simple Analytical Atmospheric Model Coupled with a Mixed-layer Ocean Model

• Free decay time scale of the coupled system depends on flux exchanges at the surface, heat loss at TOA and energy transports between tropics and mid-latitudes

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Composite Anomalies of T and SSTComposite Anomalies of T and SST

Zonal Avg. of Tropospheric Temperature Anomaly Zonal Avg. of Tropospheric Temperature Anomaly Regression on Niño3.4 SSTaRegression on Niño3.4 SSTa

Lead/Lag Regression <T>´-Niño3.4 SSTaLead/Lag Regression <T>´-Niño3.4 SSTa --comp--comp

<T>´ lags Niño-3.4 SSTa•NCEP 4 Months •OBS SST

3 Months•OBSPAC SST+ML

2 Months•CLIM+OBSPAC SST

1 Month

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Lead/Lag Regression <T>´-Niño3.4 SSTaLead/Lag Regression <T>´-Niño3.4 SSTa depth

•Phase Lag is not a monotonic function of mixed-layer ocean depth

•Amplitude of <T>´ decreases as mixed-layer ocean depth increases

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Precipitation response Precipitation response • Little lag behavior for precipitation inside or outside ENSO SST forcing regions

• Tropical-mean precipitation shows a much longer lag than tropospheric temperature

Experiments with Sinusoidal SST forcingExperiments with Sinusoidal SST forcing

QTCM Experiments with prescribedQTCM Experiments with prescribedsinusoidal SST forcingsinusoidal SST forcing

•Phase lag of <T>´ is not a monotonic function of MLD•Phase lag increases as SST forcing period increases

•Amplitude of <T>´ decreases as MLD increases•Amplitude increases as SST forcing period increases

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A Simple Analytical ModelA Simple Analytical Model

Free Decay Rate of the Coupled SystemFree Decay Rate of the Coupled System

Analytical ResultsAnalytical Results

•Analytical results are consistent with the model results•At low and high values of MLD, the lag is close to that determined by atmosphere internal heat capacity

Lead/Lag Regression <T>´-Niño3.4 SSTa Lead/Lag Regression <T>´-Niño3.4 SSTa withwith and and withoutwithout Advection Anomaly Advection Anomaly

•Lag and amplitude increase when anomalous advection of T and q are suppressed

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With and Without Advection AnomaliesWith and Without Advection Anomalies

•Lag and amplitude increase when advection anomalies suppressed

SummarySummary• The lag of <T>' relative to ENSO SST exists even when no air-sea coupling is allowed, but coupling with a slab mixed-layer ocean significantly increases the lag.

• The lag and amplitude of <T>' depend on mixed-layer ocean depth, ENSO SST forcing period and areal fraction of mixed-layer ocean.

• The behavior of phase and amplitude variations of <T>' can be explained by a simple analytical model. The free decay time scale of the coupled atmosphere-ocean system depends on various flux exchange coefficients at the surface, at the top-of-atmosphere, and between tropics and mid-latitudes. It indicates the extent of the contribution of atmosphere-ocean coupling to the lagged response of <T>' to ENSO SST.

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Coupled Atmosphere-Ocean Response Coupled Atmosphere-Ocean Response in a switch-on SST anomaly experimentin a switch-on SST anomaly experiment

• Atmosphere Fast and coupled slow modes

• Mixed-Layer Ocean SST Slow mode

• Precipitation Fast and slow varying